The present invention relates to an improved semi-trailer platform construction and notably to the construction of commercial vehicles for transporting goods.
1. Field of the Invention
The semi-trailer platform according to the present invention is so constructed as to constitute a straight frame of the semi-trailer, to which the axles and body are secured. The floor is fixed directly to the platform for supporting the goods.
2. The Prior Art
In conventional semi-trailers the platform consists as a rule of an assembly of interlocked longitudinal members and cross members and cross members made chiefly of steel, due to its good mechanical properties and relatively low cost. Another advantageous property of steel is that it is weldable and that the welds are capable of withstanding severe mechanical stresses. However, steel structures are objectionable for various reasons.
Firstly, steel has a poor corrosion resistance and must be protected by means of expensive surface treatments of limited efficiency and duration. Besides, corrosion is inasmuch important as the resistance to corrosion is an essential factor in the useful life of the frame structures. Furthermore, the detrimental effects of corrosion tend to increase with the generalization of the use of salt for de-icing roads.
Another inconvenience of steel is its weight. In fact, it is quite obvious that in the transport of goods it is more advantageous to have the lightest possible supporting structure in order to cut fuel costs. From this specific point of view, steel is rather uneconomical and therefore in direct competition with aluminum or aluminum alloys which offer many advantageous features.
Aluminum has a very good corrosion resistance even in the absence of any particular treatment, and its new alloys display considerably improved mechanical properties. Moreover, due to recent developments in welding techniques aluminum and light alloys can now be used satisfactorily in mechanically welded structures such as those used in the construction of straight frame semi-trailers.
As a matter of fact, in this specific field a rather popular type of straight frame semi-trailer platform comprises cross members interlocked with longitudinal members of which the outermost ones consist of channel sections having shaped edges and a lower stiffening flange. These section members are obtained by extrusion and as a rule the various component elements are assembled by welding.
At the present time, the frame cross members have a circular cross-section. This solution was chosen notably because it is much easier to machine a circular-sectioned member. However, in actual practice it was found that this type of cross member is attended by many inconveniences.
These inconveniences are observed more particularly in the areas where the ends of the cross members are connected to the side members where the fixing operation is difficult to perform and the stresses transmitted to the body are considerable. In fact, the lower flanges of these side members are more or less in the way when attempting to make a continuous weld bead. Under these conditions, only the upper portion of the end of the cross-member is connected to the side member. The circular cross-section of the cross member which bears on the lower flange of the longitudinal member reduces the surface of the tangent area to barely one point, hence the great fragility in this area and this is inasmuch detrimental as this point lies in a particularly stressed area.
Many failures have been recorded in this area due to the absence of any reliable bond between the lower portion of the cross member and the side member.
Another inconvenience of this circular cross-sectional contour is that its moment of inertia is far from its optimal value, considering the mass of material involved and the direction of the various stresses developed therein.